Neutron-detecting apparatuses and methods of fabrication

ABSTRACT

Neutron-detecting structures and methods of fabrication are provided which include: a substrate with a plurality of cavities extending into the substrate from a surface; a p-n junction within the substrate and extending, at least in part, in spaced opposing relation to inner cavity walls of the substrate defining the plurality of cavities; and a neutron-responsive material disposed within the plurality of cavities. The neutron-responsive material is responsive to neutrons absorbed for releasing ionization radiation products, and the p-n junction within the substrate spaced in opposing relation to and extending, at least in part, along the inner cavity walls of the substrate reduces leakage current of the neutron-detecting structure.

CROSS-REFERENCE TO RELATED APPLICATION

This application is a divisional of U.S. patent application Ser. No.14/074,131, filed Nov. 7, 2013, entitled “Neutron-Detecting Apparatusesand Methods of Fabrication”, which claims the benefit of U.S.Provisional Patent Application No. 61/723,471, filed Nov. 7, 2012, eachof which is hereby incorporated herein by reference in its entirety.

BACKGROUND

Efficient solid-state neutron-detectors with large detecting surfacesand low gamma sensitivity are desired for detecting and preventingproliferation of special nuclear materials (SNMs). Unfortunately,available neutron-detectors are limited, for instance, by size, weight,high bias voltage requirements, and/or cost due, for instance, tolimited supply of enriched helium (³He) gas, which is currently employedin most neutron-detectors.

Although a variety of solid-state neutron-detectors have been proposed,existing neutron-detectors often embody a trade-off betweenneutron-detector efficiency and gamma discrimination, as most neutronsources or reactions are generally accompanied by gamma ray events. Forexample, an increase in sensitivity of a neutron-detector often resultsin a concomitant increase in sensitivity of detecting undesired gammaray events.

Thus, there remains a need for further neutron-detection approaches, andin particular, a need for a novel, self-powered, robust and efficientsolid-state neutron-detector.

BRIEF SUMMARY

The shortcomings of the prior art are overcome and additional advantagesare provided through the provision, in one aspect, of a method whichincludes fabricating a neutron-detecting structure, the fabricatingincluding: providing a substrate including a plurality of cavitiesextending into the substrate from a surface thereof; forming a p-njunction within the substrate and extending, at least in part, in spacedopposing relation to inner cavity walls of the substrate defining theplurality of cavities therein, the p-n junction within the substratespaced in opposing relation to and extending, at least in part, alongthe inner cavity walls of the substrate reducing leakage current of theneutron-detecting structure; and providing a neutron-responsive materialwithin the plurality of cavities, the neutron-responsive material beingresponsive to neutrons absorbed thereby for releasing ionizing radiationreaction products.

Additional features and advantages are realized through the techniquesof the present invention. Other embodiments and aspects of the inventionare described in detail herein and are considered a part of the claimedinvention.

BRIEF DESCRIPTION OF THE SEVERAL VIEWS OF THE DRAWINGS

One or more aspects of the present invention are particularly pointedout and distinctly claimed as examples in the claims at the conclusionof the specification. The foregoing and other objects, features, andadvantages of the invention are apparent from the following detaileddescription taken in conjunction with the accompanying drawings inwhich:

FIG. 1A is a cross-sectional elevational view of one embodiment of astructure obtained during fabrication of a neutron-detecting structure,in accordance with one or more aspects of the present invention;

FIG. 1B depicts the structure of FIG. 1A, after electrical contacts havebeen provided over a surface thereof, in accordance with one or moreaspects of the present invention;

FIG. 1C depicts the structure of FIG. 1B, after etching thereof toprovide a plurality of cavities within the substrate, in accordance withone or more aspects of the present invention;

FIG. 1D depicts the structure of FIG. 1C, after provision of a conformallayer of material over the structure, including within the cavitiesthereof, in accordance with one or more aspects of the presentinvention;

FIG. 1E is a plan-view of one embodiment of the structure of FIG. 1D,with the plurality of cavities shown arrayed in a honeycomb pattern, andthe plurality of cavities being a plurality ofhexagonal-cross-sectional-shaped cavities, in accordance with one ormore aspects of the present invention;

FIG. 1F depicts the structure of FIG. 1D, after a continuous p-njunction has been formed within the substrate, in accordance with one ormore aspects of the present invention;

FIG. 1G depicts the structure of FIG. 1F, after optional removal of theconformal layer of material from the plurality of cavities, inaccordance with one or more aspects of the present invention;

FIG. 1H depicts the structure of FIG. 1G, after deposition ofneutron-responsive material within the cavities thereof, in accordancewith one or more aspects of the present invention;

FIG. 1I depicts the structure of FIG. 1H, after etch-back of theneutron-responsive material and provision of a contact over a secondsurface structure, in accordance with one or more aspects of the presentinvention;

FIG. 2 is a graphical representation of one embodiment of a temperatureprofile utilized during deposition of the conformal layer of material,formation of the continuous p-n junction, and subsequent deposition ofthe neutron-responsive material within the cavities, in accordance withone or more aspects of the present invention;

FIG. 3 is an enlarged depiction of the neutron-detecting structure ofFIG. 1I, showing an enlarged depletion region within the substrate dueto the presence of the continuous p-n junction, in accordance with oneor more aspects of the present invention;

FIG. 4A is a cross-sectional elevational view of another embodiment of astructure obtained during fabrication of neutron-detecting structure, inaccordance with one or more aspects of the present invention;

FIG. 4B depicts the structure of FIG. 4A, after etching thereof toprovide a plurality of cavities within the substrate, in accordance withone or more aspects of the present invention;

FIG. 4C depicts the structure of FIG. 4B, after deposition of aconformal layer of material over the structure, including within thecavities thereof, in accordance with one or more aspects of the presentinvention;

FIG. 4D depicts the structure of FIG. 4C, after a continuous p-njunction has been formed within the substrate, in accordance with one ormore aspects of the present invention;

FIG. 4E depicts the structure of FIG. 4D, after deposition ofneutron-responsive material within the cavities thereof, in accordancewith one or more aspects of the present invention; and

FIG. 4F depicts the structure of FIG. 4E, after contacts have beenprovided over opposite surfaces thereof for electrical connection to theneutron-detecting structure, in accordance with one or more aspects ofthe present invention.

DETAILED DESCRIPTION

Aspects of the present invention and certain features, advantages, anddetails thereof, are explained more fully below with reference to thenon-limiting embodiments illustrated in the accompanying drawings.Descriptions of well-known materials, fabrication tools, processingtechniques, etc., are omitted so as to not unnecessarily obscure theinvention in detail. It should be understood, however, that the detaileddescription and the specific examples, while indicating embodiments ofthe invention, are given by way of illustration only, and are not by wayof limitation. Various substitutions, modifications, additions and/orarrangements within the spirit and/or scope of the underlying inventiveconcepts will be apparent to those skilled in the art from thisdisclosure. Further, note that in making reference below to the drawings(which are not drawn to scale for ease of understanding) the samereference numbers used throughout different figures designate the sameor similar components.

Neutrons, being charge-less particles, tend not to ionize. However,their collisions with other nuclei often result in energetic ionizationreaction products, which in turn generate electron-hole pairs (EHPs),which may be separated either by a built-in electric field or by anexternal bias voltage. These electron-hole pairs can be efficientlydetected by solid-state semiconductor junctions, resulting in suchneutron-detectors being generally used in a range of applications,including, for example, civilian and defense applications.Unfortunately, available neutron-detectors are often limited, forinstance, by size, weight and high bias voltage requirements.Furthermore, the limited supply of enriched helium (3He), which iscurrently employed in many neutron-detectors, results in significantcost constraints and performance limitations.

The realization of a chip-scale, self-powered or very lowpower-consuming, efficient solid-state neutron-detector utilizingmatured silicon processing technology, such as disclosed herein, willprovide significant cost and volume benefits, as well as allowwafer-level detector integration with, for instance, charge preamplifierand/or neutron-event counting electronics.

To summarize, as an enhancement to existing detectors, disclosed hereinis an apparatus which includes a neutron-detecting structure comprising,for instance, a substrate including a plurality of cavities extendinginto the substrate from a surface thereof; a continuous p-n junctionwithin the substrate and extending, at least in part, in spaced opposingrelation to inner cavity walls of the substrate defining the pluralityof cavities; and a neutron-responsive material disposed within theplurality of cavities. The neutron-responsive material is responsive toneutrons absorbed thereby for releasing ionizing radiation reactionproducts detected by the detector, and the continuous p-n junctionwithin the substrate spaced in opposing relation to and extending, atleast in part, along the inner cavity walls of the substrateadvantageously reduces leakage current of the neutron-detectingstructure.

In one specific embodiment, the p-n junction is formed by depositing aconformal layer of p-type dopant material at a first temperature, andannealing the conformal p-type dopant material at a second temperature,which is higher than the first temperature. Annealing at the secondtemperature facilitates forming the continuous p-n junction extendingwithin the substrate in spaced opposing relation to the inner cavitywalls of the substrate. In one example, the second temperature is atleast about 100° C. to 300° C. higher than the first temperature. Notethat the conformal layer of p-type dopant material may comprise aconformal layer of neutron-responsive material deposited within theplurality of cavities. This conformal layer of neutron-responsivematerial may be, for instance, a p-type dopant such as, for example,enriched boron (¹⁰B) or a compound of enriched boron, for instance,boron carbide (¹⁰B₄C) or boron nitride (¹⁰BN).

As noted, in one embodiment, the p-n junction formed within thesubstrate is a continuous p-n junction and is disposed, in part,parallel to the surface of the substrate from which the plurality ofcavities extend into the substrate, as well as in spaced opposingrelation to the inner cavity walls of the substrate. In one specificexample, the continuous p-n junction may be opposing and spaced from thesurface of the substrate a greater distance than the continuous p-njunction is spaced from the inner cavity walls of the substrate.Advantageously, in operation, the substrate will include a depletionregion which, due to the presence of the continuous p-n junction,extends within the substrate to at least a depth of the plurality ofcavities within the substrate.

In one implementation, one or more of the plurality of cavities withinthe substrate is, at least in part, a hexagonal-cross-sectional-shapedcavity. For instance, the plurality of cavities may be arrayed, at leastin part, in a honeycomb pattern within the substrate, whichadvantageously assists with mechanical stability to the resultantsolid-state neutron-detecting structure.

In one embodiment, the apparatus may further include multipleneutron-detecting structures or modules such as disclosed herein, whichmay be advantageously electrically coupled in series. Note that thesolid-state, neutron-detecting structures disclosed herein are designedor configured to operate at minimal, or even zero, bias voltage.

By way of explanation, certain embodiments of a neutron-detectingstructure and methods of fabrication thereof, in accordance with one ormore aspects of the present invention, are described below withreference to FIGS. 1A-1I.

FIG. 1A illustrates a structure 100 attained during fabrication of asolid-state, neutron-detecting structure, in accordance with one or moreembodiments of the present invention. In the depicted embodiment,structure 100 includes a substrate 102, which as one example, may be abulk semiconductor material, such as, for example, a bulk silicon waferin a crystalline structure with any suitable crystallographicorientation. Suitable crystallographic orientations may include, forexample, (100), (110) and (111) orientation. Although not critical tothe invention, in one example, substrate 102 may have a planar (100)crystallographic surface orientation (referred to as “(100)” surface).

In the depicted example, substrate 102 has been implanted with n-typedopants to create a high-conducting n⁻ region 104, as well as an n⁻region 106. Note that, n-type dopant refers to the addition ofimpurities to, for instance, intrinsic (undoped) substrate material,which contribute more electrons to the intrinsic material, and mayinclude (for instance) phosphorus, arsenic or antimony. In one example,n⁻ region 104 and n⁻ region 106 of the substrate may be formed usingconventional ion implantation or diffusion processing techniques. The n⁺region 104 may have a thickness in the range of about 100 to 1,000microns, and n⁻ region may have a thickness of about 40 μm to 50 μm.Additionally, the resistivity of n⁻ region may be in the range of about10-50 Ω-cm. One skilled in art will understand that n⁺ region 104 ofsubstrate 102 is heavily-doped with n-type dopants as compared to n⁻region 106 of the substrate.

Substrate 102 of structure 100 further includes a highly conducting p⁺region 108 disposed over n⁻ region 106. This p⁺ region 108 may beobtained by addition of impurities to, for instance, intrinsic (undoped)substrate material to create deficiencies of valence electrons in theintrinsic material. Examples of appropriate p-type dopant include boron,aluminum, gallium, or indium. In one example, p⁺ region 108 of substrate102 is formed using conventional ion implantation or diffusionprocessing techniques and may have thickness of about 1 to 3 μm.

FIG. 1B depicts structure 100 after provision of conductive contacts 110over p⁺ region 108 of the substrate. Conductive contacts 110 may beselectively patterned as desired over p⁺ region 108, and facilitatesubsequent electrical connection to the resultant neutron-detectingstructure or module. Note that conductive contact material may be any ofa variety of conductive materials, such as tungsten, titanium, copper,aluminum, molybdenum etc. Although not depicted, one skilled in the artwill recognize that a silicide may optionally be formed by providing alayer of polysilicon over p⁺ region of the substrate, prior to thedeposition of the conductive contact material. The layer of polysiliconreacts chemically with the silicon of p⁺ region 108 to form the silicideover p⁺ region 108 of the substrate. In one example, the layer ofpolysilicon deposited over p⁺ region 108, may have thickness in therange of about 30-50 nm, while the conductive contact material depositedover the layer of polysilicon may have thickness in the range of about50 to 100 nm. Note that the layer of polysilicon, if provided, also actsas a buffer layer in preventing the diffusion of conductive contactmaterial into the underlying p⁺ region, during subsequent deviceprocessing.

A protective layer 112 may be provided over the conductive contacts 110,using, for instance, any conventional deposition processes, such asatomic layer deposition (ALD), chemical-vapor deposition (CVD), physicalvapor deposition (PVD) or the like. In one example, protective layer 112may be or include an oxide material, for instance, silicon dioxide, andmay be provided to protect the conductive contacts structure, duringsubsequent fabrication processing.

As depicted in FIG. 1C, a portion of substrate 102 is patterned with aplurality of cavities 116, which extend (in the depicted example) from asurface 118 of substrate 102 into at least a portion of n⁻ region 106 ofsubstrate 102. Note that deep reactive ion etching (DRIE) or plasmaetching process may be employed to pattern substrate 102 with aplurality of high-aspect-ratio cavities 116. In another example, ananisotropic dry etching process may alternatively (or also) be employedto pattern the cavities. In one specific example, deep reactive ionetching is performed using fluorine-based chemistry, which may involveprocess gases such as nitrogen trifluoride (NF₃), sulfur hexafluoride(SF₆), tetrafluoromethane (CF₄), trifluoromethane (CH₃F),difluoromethane (CH₂F₂), fluoromethane (CH₃F), octafluorcyclobutane(C₄F₈), hexafluoro-1,3-butadiene (C₄F₆) in inert gaseous medium such asargon (Ar).

In one specific example, one or more, or even each, cavity of theplurality of cavities 116 is configured with a hexagonal-cross-sectionalshape. By way of example, the hexagonal-cross-sectional-shaped cavitiesmay have a diameter in the range of about 1-3 μm and a depth of about40-50 μm, extending into the substrate, with adjacent cavities beingseparated, for example, by about 1 to 1.3 μm of substrate.

As illustrated in FIG. 1D, a conformal layer 122 of material (in thisexample, p-type dopant material) may next be deposited. Conformal layer122, which overlies structure 100, including within the plurality ofcavities 116, may be deposited using a modified chemical vapordeposition (CVD) process. For instance, the CVD process may be modifiedby varying parameters such as, temperature and pressure, to obtain thedesired conformal layer. Note that the conformal layer 122 of p-typedopant material may be deposited at a first temperature, for instance,in the range of about 450° C. to 550° C.

Conformal layer 122 may be or include, in one embodiment, a conformallayer of neutron-responsive material, which may be or include the p-typedopant. Examples of appropriate p-type dopants include boron, aluminum,gallium, or indium, being deposited. In one specific example, theconformal layer of neutron-responsive material may include at least oneof enriched boron (¹⁰B) or a compound of enriched boron such as, forexample, boron carbide (¹⁰B₄C, ¹⁰B₅C) or boron nitride (¹⁰BN).

In one specific example, conformal layer 122 may be deposited using aconventional CVD process, by employing enriched boron precursors suchas, for example, diborane (B₂H₆), deca-borane(B₁₀H₁₄) or other metalorganoborane precursors such as, triethylborane(C₂H₅)₃B ortrimethylborane (CH₃)₃B, at about 500° C. Note that the enriched boronprecursors employed may contain more than 95% of enriched boron (¹⁰B)isotope. In one embodiment, thickness of the conformal layer along theinner walls of the cavities 116 may be, for example, in the range ofabout 10 to 20 nm. Note that the conductive contacts 110, discussedabove in connection with FIG. 1B, remain unaffected by this processing,due to the low temperature conditions employed. One skilled in the artwill also note that any residual layer of polysilicon that may have beendeposited over p⁺ region 108 of substrate 102, during the formation ofconductive contacts 110, may be converted to silicide, under these lowtemperature deposition conditions, by reacting chemically with anyresidual underlying silicon of p⁺ region 108, thereby further improvingthe quality of the conductive contacts 110.

By way of example, FIG. 1E is a partial cross-sectional plan view of oneembodiment of a neutron-detection structure, such as described above inconnection with FIGS. 1A-1D. As illustrated, in one or more embodiments,the plurality of cavities 116 of the neutron-detecting structure ormodule may be arrayed in a honeycomb pattern within the substrate 112.For instance, one or all the plurality of cavities may have ahexagonal-cross-sectional shape, with the resultant honeycomb patternproviding the solid-state, neutron-detecting structure with enhancedmechanical rigidity compared with other cavity configurations andlayouts.

As illustrated in FIG. 1F, conformal layer 122 (of p-type dopantmaterial) is subjected to a controlled annealing process to provide ap-region 124 within substrate 102 along the inner walls of the pluralityof cavities 116. The result is to form a continuous p-n junction 125within substrate 102 at the interface between p⁺ region 108, n⁻ region106, and between p-region 124 and n⁻ region 106. Note that thecontrolled annealing to produce p-region 124 is performed at a secondtemperature, higher than the first temperature. This higher temperatureanneal results in a portion of the p-type dopant from conformal layer122 diffusing into the underlying n⁻ region 106 of the substrate,thereby facilitating formation of the continuous p-n junction 125 withinthe substrate.

Note that the second, annealing temperature is at least about 100° C. to300° C. higher than the first temperature at which conformal layer 122is deposited. As one specific example, continuous p-n junction 125 maybe formed within substrate 102, by increasing the process temperaturefrom 500° C. to about 700° C., for 10 to 30 minutes, to promotediffusion of p-type dopant (such as, for example, boron) from theconformal layer into the underlying n⁻ region 106 of the substrate. Thethickness of p-region 124 may be controlled by controlling processparameters such as, for instance, temperature and time, at which theannealing is performed. In one example, the thickness of p-region 124may be in the range of about 20 nm to 200 nm. In the embodimentillustrated, continuous p-n junction 125 is, in part, in spaced opposingrelation to surface 118 of the substrate, and is, in part, in spacedopposing relation to the inner walls of substrate 102 defining cavities116. As illustrated, continuous p-n junction 125 is spaced from surface118 of the substrate 102 a greater distance than it is spaced from theinner cavity walls of the substrate. In one embodiment, conformal layer122 may be polished back from upper surface of structure 100 using, forinstance, chemical mechanical polishing, stopping on contacts 110 and p⁺region 108 of the substrate. Note that the conformal layer of p-typedopant material disposed within cavities 116 would remain unaffected,during this etch-back polishing processing. After this polish-back,neutron-responsive material may be disposed within the cavities 116 ofstructure 100, in a manner such as described below.

FIG. 1G depicts an alternate embodiment, wherein conformal layer 122 isremoved from structure 100 after formation of p-region 124 within thesubstrate. By way of example, conformal layer 122 may be selectivelyremoved using, for instance, a plasma formed from, for example, amixture of oxygen (O₂) and a fluorine-containing gas, such as, carbontetrafluoride (CF₄). In FIGS. 1H & 1I depicted below, it is assumed thatthe conformal layer 122 has been removed prior to provision of theneutron-responsive material within the plurality of cavities. As notedabove, in an alternate implementation, the conformal layer 122 mayremain within the plurality of cavities without affecting operation ofthe resultant neutron-detecting structure, particularly where theconformal layer comprises the above-noted enriched boron (¹⁰B) orcompounds of enriched boron.

FIG. 1H illustrates structure 100 after a neutron-responsive material126 has been provided within cavities 116. In one embodiment,neutron-responsive material 126 may be or include at least one ofenriched boron (¹⁰B) or a compound of enriched boron such as, forexample, boron carbide (¹⁰B₄C, ¹⁰B₅C) or boron nitride (¹⁰BN). In onespecific example, neutron-responsive material 126 may be deposited usinga low-temperature, high-pressure CVD process, by employing enrichedboron precursors such as, for example, diborane (B₂H₆),deca-borane(B₁₀H₁₄) or other metal organoborane precursors such as,triethylborane(C₂H₅)₃B or trimethylborane (CH₃)₃B, at about 500° C. Notethat the enriched boron precursors employed herein may contain, forinstance, more than 95% of enriched boron (¹⁰B) isotope. Note also thatthe neutron-responsive material (for instance, enriched boron or acompound of enriched boron) advantageously facilitates absorbing thermalneutrons and converting the absorbed neutrons into energetic chargedparticles, thereby allowing for the detection operation of thesolid-state, neutron-detecting structure. In one example, enriched boronor a compound of enriched boron has a high thermal absorptioncoefficient, for instance, of about 3840 barn, making enriched-boron anefficient neutron-responsive material. Additionally, large absorptionlengths of neutrons in boron-rich neutron-detectors, and short escapelengths of energetic-charged particles, further enhance the efficiencyof boron-rich neutron-detecting structures.

Note further that, in an alternate embodiment, neutron-responsivematerial 126 may comprise other materials capable of performing theneutron-detection function. For instance, the material may alternativelybe or include a hydrogen-rich aromatic polymer material such as, forexample, polyp-xylylene) polymer (also referred to herein as parylene)or polystyrene.

Note that the low-temperature, high-pressure chemical vapor depositionprocess employed to deposit the neutron-responsive material withincavities 116 advantageously facilitates filling the cavities withoutdefects, and thereby, enhances the efficiency of the resultantneutron-detecting structure. Although the modified deposition conditionsaccomplish an efficient filling of the cavities, one skilled in the artwill note that with high-aspect-ratio depositions, a tear-shaped voidmay be created within one or more of the cavities. In the event of suchan occurrence, a small portion of the neutron-responsive material may beetched using any suitable etching processing, for example, reactive ionetching, while protecting the remaining neutron-responsive materialwithin the cavities, for instance, using a photoresist material, andsubsequently be re-deposited until the tear-shaped void is removed.Alternatively, one or more small voids within the cavities may remain inplace without significantly affecting operation of the resultantneutron-detecting structure.

As depicted in FIG. 1I, the neutron-responsive material 126 may bepartially removed to expose conductive contacts 110 on the one side ofthe structure, and a conductive contact 128 may be provided on theopposite side, for instance, the under-side of the structure. In oneembodiment, conductive contact 128 is provided over highly conducting n⁺region 104 of substrate 102. Note that conductive contact 128 may beformed of any of a variety of conductive materials, such as tungsten,titanium, copper, aluminum, molybdenum etc. Although not depicted, oneskilled in the art will recognize that a silicide may also be formed,for instance, by providing a layer of polysilicon over the exposedsurface of n⁺ region 104, prior to the deposition of the conductivecontact. The layer of polysilicon reacts chemically with the silicon ofn⁺ region 104 to form silicide over the n⁺ region. In one example, thelayer of polysilicon deposited over n⁺ region 104 may have thickness inthe range of about 30-50 nm, while conductive contact 128 deposited overthe layer of polysilicon may have thickness in the range of about 50 to100 nm. Note that, if provided, the polysilicon will also act as abuffer layer in preventing the diffusion of conductive contact 128 intothe underlying n⁺ region during subsequent fabrication processing.

By way of further example, FIG. 2 is a graphical representation of atemperature profile which may be employed during device fabrication,including deposition of a conformal layer of material, formation of thecontinuous p-n junction and a subsequent deposition of theneutron-responsive material within the cavities, in accordance with oneor more aspects of the present invention. As discussed, in oneembodiment, a conformal layer of p-type dopant material may be deposited(A), employing a low-temperature, high-pressure chemical-vapordeposition (CVD) process. As one specific example, a p-typeneutron-responsive material such as, boron may be deposited within thecavities at about 500° C. The conformal layer may be subjected to anannealing process (B) by increasing the process temperature by about100° C. to 300° C. for a short time duration, for example, about 10 minsto 30 mins, resulting in diffusion of a portion of p-type material (forexample, boron) into the underlying substrate, thereby forming theportion of the p-n junction within the substrate extending, at least inpart, in spaced opposing relation to the inner cavity walls of thesubstrate. The neutron-responsive material may subsequently be deposited(C) within the cavities, during which the temperature of the CVD processis lowered to about 500° C. and the pressure is increased by about 100ton, as compared to a conventional CVD process. Note that these modifiedprocess parameters efficiently improve the deposition rate of theneutron-responsive material within the cavities. In one specificexample, the deposition rate at which the neutron-responsive material isdeposited may be in the range of about 1.5 to 2₁m/hr. Note thatefficiency of the resultant neutron-detecting structure may depend onthe particular process parameters, such as temperature and pressure,used during chemical-vapor deposition of the neutron-responsivematerial.

The efficiency of neutron-detecting structures may be further enhance bypatterning a silicon substrate having (110) crystallographicorientation, with a plurality of cavities using, for example,highly-selective, conventional wet-etching processes. In such anexample, the inner cavity walls may have (111) and (100)crystallographic orientations, and a controlled, low-pressure CVDprocess may be employed to deposit the neutron-responsive material.

As an operational example, FIG. 3 depicts an enlarged partial view ofthe structure of FIG. 1I. In this example, an increased depletion region300 (i.e., compared with the depletion region of a conventionalneutron-detector) is formed within the substrate due to the presence ofthe continuous p-n junction 125 wrapping around the cavities. Note thatcontinuous p-n junction 125 may act as a passivation layer along theinner walls of cavities 116, as well as provide a built-in electricfield in the radial direction which completely depletes the adjoininginner walls of the substrate without any external biasing voltage,thereby increasing the area of the depletion region within thesubstrate. Passivation of the substrate using continuous p-n junction125, and the increased size of depletion region 300 within thesubstrate, advantageously facilitate reducing leakage current andcapacitance of the resultant neutron-detecting structure, and therebyfacilitate producing devices with large collection surface areas, andimproved charge collection efficiency, with minimal external biasvoltage required. As noted, the continuous p-n junction may even operateat zero bias voltage.

Those skilled in the art will note that the reverse leakage current I₀of the detector depends on diffusion current and recombination current.The diffusion current is given by:

${I_{od} = {{qA}\; \frac{D_{p}n_{i}^{2}}{L_{p}N_{D}}}},$

where I_(od) is the diffusion part of the reverse leakage current, A isthe detector area, D_(p) and L_(p) are the diffusion coefficient anddiffusion length of the n-type silicon respectively, n_(i) is theintrinsic carrier concentration for intrinsic silicon, and ND is thedoping concentration of n-type silicon respectively.

The recombination current is given by:

${I_{or} = \frac{{qn}_{i\mspace{11mu} {AW}}}{2\tau_{o}}},$

where W is the depletion layer width, and τ₀ is approximately an averageof electron and hole lifetime and depends on the location of therecombination center in the bandgap.

As noted, the continuous p-n junction disclosed herein advantageouslyfacilitates in preventing the n⁻ region from being exposed (except atthe edge of the neutron-detector), thereby resulting in a residualleakage current principally due to any recombination current. Note alsothat the n⁻ region remains unaffected further reducing the diffusioncurrent, and the depletion region extends along the inner walls of theplurality of cavities, thereby further reducing the leakage current.

By way of further example, FIGS. 4A-4F depicts another embodiment of aneutron-detecting structure and methods of fabrication thereof, inaccordance with one or more aspects of the present invention.

FIG. 4A illustrates a structure 400 attained during fabrication of asolid-state neutron-detecting structure, in accordance with one or moreaspects of the present invention. In this embodiment, structure 400includes a substrate 402, which may be a bulk semiconductor material,such as, for example, a bulk silicon wafer in a crystalline structurewith any suitable crystallographic orientation. Suitablecrystallographic orientations may include, for example, (100), (110) and(111) orientation. Although not critical to the invention, in oneexample, substrate 402 may have a planar (100) crystallographic surfaceorientation (referred to as “(100)” surface).

Substrate 402 has been implanted with n-type dopants to create ahigh-conducting n⁺ region 404, as well as an n⁻ region 406. Note that,n-type dopant refers to the addition of impurities to, for instance,intrinsic (undoped) substrate material, which contribute more electronsto the intrinsic material, and may include (for instance) phosphorus,arsenic or antimony. In one example, n⁺ region 404 and n⁻ region 406 ofthe substrate may be formed using conventional ion implantation ordiffusion processing techniques and the n⁺ region 404 may have athickness in the range of about 1 to 3, and n⁻ region may have athickness of about 40 μm to 50 μm. Additionally, the resistivity of n⁻region may be in the range of about 10-50 a-cm. One skilled in art willunderstand that the n⁺ region 404 of substrate 402 is heavily-doped withn-type dopants as compared to n⁻ region 406 of the substrate.

Substrate 402 of structure 400 further includes a high-conducting p⁺region 408 disposed over n⁻ region 406. This p⁺ region 408 may beobtained by addition of impurities to, for instance, intrinsic (undoped)substrate material to create deficiencies of valence electrons in theintrinsic material. Examples of appropriate p-type dopant may includeboron, aluminum, gallium, or indium. In one example, p⁺ region 408 ofsubstrate 402 is formed using conventional ion implantation or diffusionprocessing techniques and may have thickness of about 1 to 3 μm.

As depicted in FIG. 4B, a portion of substrate 402 may be patterned witha plurality of cavities 416, which extend (in the depicted example) froma surface 418 of substrate 402 into at least a portion of n⁻ region 406of substrate 402. Note that a deep reactive ion etching (DRIE) or plasmaetching may be employed to pattern substrate 402 with a plurality ofhigh-aspect-ratio cavities 416. In another example, an anisotropic dryetching process may alternatively (or also) be employed to pattern thecavities. In one specific example, deep reactive ion etching isperformed using fluorine-based chemistry, which may involve processgases such as nitrogen trifluoride (NF₃), sulfur hexafluoride (SF₆),tetrafluoromethane (CFO, trifluoromethane (CH₃F), difluoromethane(CH₂F₂), fluoromethane (CH₃F), octafluorcyclobutane (C₄F₈),hexafluoro-1,3-butadiene (C₄F₆) in inert gaseous medium such as argon(Ar).

In one specific example, one or more, or even each cavity of theplurality of cavities 416 is configured with a hexagonal-cross-sectionalshape. By way of example, the hexagonal-cross-sectional-shaped cavitiesmay have a diameter in the range of about 1-3 μm and a depth of about40-50 μm, extending into the substrate, with adjacent cavities beingseparated, for example, by about 1 to 1.3 μm of substrate.

As illustrated in FIG. 4C, a conformal layer 422 of material may next bedeposited. Conformal layer 422, which overlies structure 400, includingwithin the plurality of cavities 416, may be deposited using a modifiedchemical vapor deposition (CVD) process. For instance, the CVD processmay be modified by varying parameters such as temperature and pressure,to obtain the desired conformal layer. Note that the conformal layer 422(e.g., of a p-type dopant material) may be deposited at a firsttemperature, for instance, in the range of about 450° C. to550° C.Conformal layer 422 may be or include, in one embodiment, a conformallayer of neutron-responsive material, which may be or include the p-typedopant. Examples of appropriate p-type dopant include boron, aluminum,gallium, or indium. In one specific example, the conformal layer ofneutron-responsive material may include at least one of enriched boron(¹⁰B) or a compound of enriched boron such as, for example, boroncarbide (¹⁰B₄C, ¹⁰B₅C) or boron nitride (¹⁰BN).

In one specific example, conformal layer 422 may be deposited using aconventional CVD process, by employing enriched boron precursors suchas, for example, diborane (B₂H₆), deca-borane(B₁₀H₄) or other metalorganoborane precursors such as, triethylborane(C₂H₅)3B ortrimethylborane (CH₃)₃B, at about 500° C. Note that the enriched boronprecursors employed may contain more than 95% of enriched boron (¹⁰B)isotope. In one embodiment, thickness of the conformal layer along theinner walls of cavities 416 may be, for example, in the range of about10 to 20 nm.

As illustrated in FIG. 4D, conformal layer 422 (of p-type dopantmaterial) is subjected to a controlled annealing process to provide ap-region 424 within substrate 402 along the inner walls of the pluralityof cavities 416. The result is to form a continuous p-n junction 425within substrate 402 at the interface, between p⁺ region 408 and n⁻region 406, and between p-region 424 and n⁻ region 406. Note that thecontrolled annealing to produce p-region 424 is performed at a secondtemperature, higher than the first temperature. This higher temperatureanneal results in a portion of the p-type dopant from conformal layer422 diffusing into underlying n⁻ region 406 of the substrate, therebyfacilitating formation of the continuous p-n junction 425 within thesubstrate.

Note that the second, annealing temperature is at least about 100° C. to300° C. higher than the first temperature at which conformal layer 422is deposited. As a specific example, a continuous p-n junction 425 maybe formed within substrate 402, by increasing temperature from 500° C.to about 700° C., for about 10 mins to 30 mins, to promote diffusion ofp-type dopant (such as, for example, boron) from the conformal layerinto the underlying n⁻ region 406 of the substrate. The thickness ofp-region 424 may be controlled by controlling process parameters suchas, for instance, temperature and time, at which the annealing isperformed. In one example, the thickness of p-region 424 may be in therange of about 20 nm to 200 nm. As illustrated, continuous p-n junction425 is, in part, in a spaced opposing relation to surface 418 of thesubstrate, and is, in part, in spaced opposing relation to the innerwalls of substrate 402 defining cavities 416. In the embodimentillustrated, continuous p-n junction 425 is spaced from surface 418 ofsubstrate 402 a greater distance than it is spaced from the inner cavitywalls of the substrate. In one embodiment, conformal layer 422 may bepolished back from upper surfaces of structure 400 using, for instance,chemical mechanical polishing, stopping on p⁺ region 408 of thesubstrate. Note that the conformal layer of p-type material disposedwithin cavities 416 remains unaffected, during this etch-back polishingprocess.

FIG. 4E illustrates structure 400 after a neutron-responsive material426 has been provided within cavities 416. In one embodiment,neutron-responsive material 426 may be or include at least one ofenriched boron (¹⁰B) or a compound of enriched boron such as, forexample, boron carbide (¹⁰B₄C, ¹⁰B₅C) or boron nitride (¹⁰BN). In onespecific example, neutron-responsive material 426 may be deposited usinga low-temperature, high-pressure CVD process, by employing enrichedboron precursors such as, for example, diborane (B₂H₆),deca-borane(B₁₀H₁₄) or other metal organoborane precursors such as,triethylborane(C₂H₅)₃B or trimethylborane (CH₃)₃B, at about 500° C. Notethat the enriched boron precursors employed herein may contain, forinstance, more than 95% of enriched boron (¹⁰B) isotope. Note also thatthe neutron-responsive material (for instance, enriched boron or acompound of enriched boron) advantageously facilitates absorbing thermalneutrons and converting the absorbed neutrons into energetic chargedparticles, thereby allowing for the detection operation of thesolid-state, neutron-detecting structure.

Note further that, in an alternate embodiment, neutron-responsivematerial 426 may comprise other materials capable of performing theneutron-detection function. For instance, the material may alternativelybe or include a hydrogen-rich aromatic polymer material such as, forexample, polyp-xylylene) polymer (also referred to herein as parylene)or polystyrene.

Note that the low-temperature, high-pressure chemical vapor depositionprocess employed to deposit the neutron-responsive material withincavities 416 advantageously facilitates filling the cavities withoutdefects and thereby, enhances the efficiency of the resultantneutron-detecting structure. Although the modified deposition conditionsaccomplish an efficient filling of the cavities, one skilled in the artwill note that with high-aspect-ratio depositions, a tear-shaped voidmay be created within one or more of the cavities. In the event of suchan occurrence, a small portion of the neutron-responsive material may beetched using any suitable etching processing, for example, reactive ionetching, while protecting the remaining neutron-responsive materialwithin the cavities, for instance, using a photoresist material, andsubsequently be re-deposited until the tear-shaped void is removed.Alternatively, one or more small voids within the cavities may remain inplace without significantly affecting operation of the resultantneutron-detecting structure.

As depicted in FIG. 4F, the neutron-responsive material 426 may beetched back or partially removed and conductive contacts 427, 428 may beprovided on opposite sides of the structure. Note that, in oneembodiment, conductive contact 427 resides over high-conducting p⁺region 408, and conductive contact 428 resides over the opposite side,that is, over high-conducting n⁻ region 404 of the substrate. Note alsothat conductive contacts 427, 428 may be formed of any of a variety ofconductive materials, such as tungsten, titanium, copper, aluminum,molybdenum etc. Although not depicted, one skilled in the art willrecognize that a silicide may be formed, for instance, by providingpolysilicon over p⁻ region 408 and over n⁺ region 404 of the substrate,prior to the deposition of the conductive contacts. As noted above,polysilicon reacts chemically with the silicon of p⁺ region 408 and thesilicon of n⁺ region 404 to form silicide over the p⁺ region and the n⁺region. In one example, the polysilicon deposited over p⁺ region 408 andn⁺ region 404, may have thickness in the range of about 30-50 nm, whileconductive contacts 427, 428 may have thickness in the range of about 50to 100 nm. Note that, if provided, the polysilicon will also act as abuffer layer in preventing the diffusion of conductive contacts 427, 428into the underlying p⁺ and n⁺ regions, during subsequent fabricationprocessing.

Those skilled in the art will note from the above discussion thatneutron-detector structures are provided herein with reduced leakagecurrent. These neutron-detector structures with reduced leakage currentadvantageously enable large collection surfaces to be fabricatedemploying, for instance, a single preamplifier, by electricallyconnecting multiple neutron-detector structures or modules in series.Further, formation of a continuous p-n junction within theneutron-detector structure disclosed enables the three-dimensional,micro-structured, solid-state, neutron-detector to be operated atminimal or zero bias voltage, thereby providing a low-power consumptiondetector, and a low-noise detection capability, which significantlysimplifies electronic circuit design. In one example, the thermalneutron-detection efficiency and gamma-to-neutron sensitivity wereobserved to be about 26±0.5% and (1.1±0.1)×10⁻⁵, respectively.Additionally, in certain embodiments, the plurality of cavities is aplurality of hexagonal-cross-sectional-shaped cavities arrayed in ahoneycomb pattern, which facilitates absorbing thermal neutrons in aradial direction. The honeycomb pattern also avoids any possiblestreaming effects, as well as assisting in mechanical stability ofneutron-detecting structure. As noted, multiple neutron-detectingstructures may be electrically coupled in series to reduce the overallcapacitance of the neutron-detecting sensor, and in turn, reduce noise.The detectors and fabrication methods disclosed herein facilitateneutron-detecting sensors with large detection surface areas.

The terminology used herein is for the purpose of describing particularembodiments only and is not intended to be limiting of the invention. Asused herein, the singular forms “a”, “an” and “the” are intended toinclude the plural forms as well, unless the context clearly indicatesotherwise. It will be further understood that the terms “comprise” (andany form of comprise, such as “comprises” and “comprising”), “have” (andany form of have, such as “has” and “having”), “include” (and any formof include, such as “includes” and “including”), and “contain” (and anyform contain, such as “contains” and “containing”) are open-endedlinking verbs. As a result, a method or device that “comprises”, “has”,“includes” or “contains” one or more steps or elements possesses thoseone or more steps or elements, but is not limited to possessing onlythose one or more steps or elements. Likewise, a step of a method or anelement of a device that “comprises”, “has”, “includes” or “contains”one or more features possesses those one or more features, but is notlimited to possessing only those one or more features. Furthermore, adevice or structure that is configured in a certain way is configured inat least that way, but may also be configured in ways that are notlisted.

The corresponding structures, materials, acts, and equivalents of allmeans or step plus function elements in the claims below, if any, areintended to include any structure, material, or act for performing thefunction in combination with other claimed elements as specificallyclaimed. The description of the present invention has been presented forpurposes of illustration and description, but is not intended to beexhaustive or limited to the invention in the form disclosed. Manymodifications and variations will be apparent to those of ordinary skillin the art without departing from the scope and spirit of the invention.The embodiment was chosen and described in order to best explain theprinciples of one or more aspects of the invention and the practicalapplication, and to enable others of ordinary skill in the art tounderstand one or more aspects of the invention for various embodimentswith various modifications as are suited to the particular usecontemplated.

What is claimed is:
 1. A method comprising: fabricating aneutron-detecting structure, the fabricating comprising: providing asubstrate comprising a plurality of cavities extending into thesubstrate from a surface thereof; forming a p-n junction within thesubstrate and extending, at least in part, in spaced opposing relationto inner cavity walls of the substrate defining the plurality ofcavities therein; providing a neutron-responsive material within theplurality of cavities, the neutron-responsive material being responsiveto neutrons absorbed thereby for releasing ionization radiation reactionproducts, wherein the p-n junction within the substrate spaced inopposing relation to and extending, at least in part, along the innercavity walls of the substrate reduces leakage current of theneutron-detecting structure; wherein the p-in junction within thesubstrate is a continuous p-n junction, the continuous p-n junctionbeing disposed, at least in part, parallel to the surface of thesubstrate from which the plurality of cavities extend into thesubstrate, as well as in spaced opposing relation to the inner cavitywalls of the substrate; and wherein the continuous p-n junction isspaced from the surface of the substrate a greater distance than thecontinuous p-n junction is spaced in opposing relation to the innercavity walls of the substrate.
 2. The method of claim 1, wherein formingthe p-n junction comprises depositing a conformal layer of p-type dopantmaterial at a first temperature, and subsequently annealing theconformal layer of p-type dopant material at a second temperature, thesecond temperature being higher than the first temperature, and theannealing facilitating forming a continuous p-n junction extending, atleast in part, within the substrate in spaced opposing relation to theinner cavity walls of the substrate.
 3. The method of claim 2, whereinthe second temperature is at least about 100° C. to 300° C. higher thanthe first temperature.
 4. The method of claim 2, wherein the conformallayer of p-type dopant material comprises a conformal layer ofneutron-responsive material deposited within the plurality of cavities,the conformal layer of neutron-responsive material comprising at leastone of enriched boron or a compound including enriched boron.
 5. Themethod of claim 2, wherein the continuous p-n junction is deposited, inpart, in spaced opposing relation to the surface of the substrate, thecontinuous p-n junction being spaced from the surface of the substrate agreater distance than the continuous p-n junction is spaced from theinner cavity walls of the substrate.
 6. The method of claim 1, whereinat least one cavity of the plurality of cavities within the substrateis, at least in part, a hexagonal-cross-sectional-shaped cavity.
 7. Themethod of claim 1, wherein providing the substrate further comprisesarraying, at least in part, the plurality of cavities in the substratein a honeycomb pattern.
 8. The method of claim 1, wherein fabricatingthe neutron-detecting structure comprises fabricating theneutron-detecting structure to operate at zero bias voltage.
 9. Themethod of claim 1, wherein the neutron-responsive material within theplurality of cavities comprises a hydrogen-rich aromatic polymermaterial.
 10. The method of claim 1, wherein forming the p-n junctionwithin the substrate comprises disposing a conformal layer of materialover the substrate and within the plurality of cavities extendingtherein, and annealing the conformal layer of material to form, at leastin part, the p-n junction within the substrate.
 11. The method of claim10, further comprising removing the conformal layer of material from theplurality of cavities after the annealing and before the providing ofthe neutron-responsive material within the plurality of cavities.